The present invention relates to an improved braking system which is particularly suitable for use with a powered rotary tool of the type in which a shaft is rotated about an axis by a motor, for example a lawn mower of the rotary type in which a blade and optionally an impeller are rotated about a common axis by a motor, which may be an electric motor or a petrol motor.
For safety reasons, it is necessary to incorporate a braking system within the lawn mower, so that the blade and, where provided, the impeller are brought to a stop within a specified time. With the increased concern for safety, safety requirements are continually becoming more stringent, and it is now generally accepted that a stop time of less than 3 seconds is particularly desirable, and indeed is increasingly being required by statutory regulations in many countries.
Braking systems which are currently available for use on mowers of this type include electromechanical systems which are spring loaded brake systems comprising a brake pad and a brake disc, the spring urging the pad and disc into braking contact and being opposed by the action of a solenoid.
For mowers of the type driven by induction motor brakes of the type in which the magnetic field created in the rotor is used to hold a movable brake disc in an OFF position against the spring force while the motor is running have been proposed. In these systems, the magnetic field is energised only when the motor is energised.
Braking systems for electric motors are also known, which comprise an electromagnetically releasable spring actuated brake with a braking surface, a brake disc driven in rotation, a release device non-rotatably secured to the braking surface, and an armature mounted non-rotatably and so as to be capable of axial displacement between the brake disc and the release device, and biased by springs against the brake disc. Such braking systems have been disclosed in connection with motors having substantially horizontally mounted axles to be stopped.
In such braking systems, on energising the motor the axle moves to separate the brake disc from its corresponding braking surface. On de-activating the motor, the spring urges the brake disc back into contact with the braking surface.
It is a disadvantage of the known electromechanical braking systems that they are expensive to manufacture and add significantly to the cost of each unit to which they are fitted. Where attempts have been made to reduce the cost of these systems by using part of the motor housing to provide part of the housing for the solenoid coil and spring, these systems only achieve a stop time in the order of 5 seconds, which is not sufficiently short to satisfy the more stringent safety requirements which require a stop time of less than 3 seconds.
Known electromagnetically releasable spring actuated braking systems would not be suitable for use with a substantially vertical shaft, since the force generated by the magnetic field is not sufficient to hold the armature in its raised position under normal running conditions, after the initial surge observed when the motor is energised.
It is an object of the present invention to provide a braking system suitable for a powered rotary tool, in which the above disadvantages are reduced or substantially obviated.